It is known to use casting to produce a wide range of components with complex shapes that would be otherwise difficult or uneconomical to manufacture by other methods. Molten material is poured into a mould which defines the shape of the component. The material is then allowed to cool and solidify in the shape of the mould. Where the material has a melting point well above standard ambient temperature and pressure (SATP) (which is typical for most metals), the pouring of the molten material takes place within a furnace. It is known to control the cooling of the molten material in the mould to control the microstructure of the solidified material.
It is known to provide multiple components simultaneously by arranging a plurality of moulds in a single assembly. The moulds are connected by a tree-like network of casting channels through which molten material from a casting cup can be fed to the multiple moulds simultaneously.
In, for example, turbine blades it is desirable to provide a single crystal component. This is achieved through a process of “directional solidification” wherein control is exerted over the nucleation and the growth of single crystals in a molten metal as it passes from its liquid state to a solid state. Once filled, the moulds are collectively drawn from the furnace in a controlled manner.
FIG. 1 shows in schematic a known apparatus for the simultaneous manufacture of multiple cast components using a directional solidification process. As shown in the figure the apparatus comprises a pouring cup 1 into which molten material M is poured. A plurality of feed channels 2 extend radially around the centrally arranged cup 1 to the top end of the moulds 3. Molten material M poured into the cup 1 flows along the feed channels 2 and into the moulds 3. Each mould 3 is provided with a seed crystal 4 at a bottom end. Beneath the bottom end of the moulds 3 is a chill plate 5 which is maintained generally at a temperature below the melting point of the material M creating a temperature gradient from the bottom to the top of the moulds 3. The moulds are enclosed by a heat source 6 which encircles the cup 1 and mould 3 assembly. With the moulds filled, the assembly is drawn in a controlled manner out of the heat source in the direction of arrow A to ensure directional solidification from the bottom of the moulds 3 (where the seed crystal 4 is arranged) to the top of the moulds 3. The assembly passes through a baffle 7 which separates the hot zone in which the metal is introduced from a cold zone. The combination of a single crystal seed 4 with the controlled cooling encourages growth of a single crystal structure in the semi-molten casting. The skilled addressee will be familiar with alternative single crystal starters which can be used in place of the seed crystal
Moulds for the described apparatus may be formed using the so called “lost pattern” or “investment casting” method (though other methods may be used). In this method, a pattern of the desired component shape is formed from a wax or other material of low melting point. The pattern is coated in ceramic slurry which is subsequently dried and fired to form a ceramic shell around the pattern. The pattern can then be removed from the shell, for example by melting and pouring or leaching, to provide a shell mould, the cavity of which defines the desired component shape.
Two critical aspects of the directional solidification process are the mould thickness and the clearance between the exposed surface of the mould and the baffle. Both of these aspects are difficult to control. In the case of the mould thickness, it is very difficult to get accurate measurement data of this without destroying the mould. To avoid baffle/mould interaction, the baffle must be designed to provide a minimum clearance which allows free passage of the mould at its maximum plan view dimensions through the baffle. It is difficult to get data on this maximum plan view profile of the moulds to generate an optimum baffle profile.
It is known to design baffles by reference to a CAD model of the wax patterns used to produce the shell moulds. The baffle profile is thus designed to follow the profile of the wax patterns with a separation between the baffle and the wax profiles selected to be large enough to ensure clear passage of the outer surface of the shell moulds through the baffle. This relies on a best guess as to the maximum dimension of the shell mould in a plan view. Hence, the baffle is typically designed with a larger gap than might actually be needed resulting in below optimal cooling and consequent metallurgical scrap rates which can be more than 15% of throughput.
The present invention aims to provide an apparatus which facilitates more effective cooling of molten material during a directional solidification process by comparison to the described prior art methods.